The present invention relates to a process for sampling and analyzing at least one mixture of substances by slab chromatography or thin layer chromatography, which consists in depositing on the chromatography plate an appropriate amount of the mixture to be analyzed, in bringing the chromatography plate into contact with a solvent or a mixture of solvents so as to cause the migration of the mixture on the plate and the separation of the constituents of the said mixture, and in developing the said plate by means, for example chemical or physicochemical means, for the qualitative and/or quantitative analysis of the constituents of the mixture.
The abovementioned method is well known and widely used in the laboratory. Slab chromatography or thin layer chromatography generally uses a glass plate on which a thin layer, having a thickness of 100 to 300 microns, of an absorbent substance such as, for example, silica is deposited. A drop of the mixture to be examined is deposited on this chromatography plate; it is then left to dry; the chromatography plate is then placed inside a tank, for example virtually, with its base dipping in a trough containing a solvent or a mixture of solvents, which rises up the plate by capillary action. The various constituents of the mixture to be analyzed will migrate to a greater or lesser extent along a vertical line and produce spots which will then be developed. The position of the spots makes it possible to determine the nature of each of the constituents.
The mixture to be subjected to analysis may be present on a substrate. This is the case, for example, with the sebum secreted by the skin, and it is desirable, in dermatology, to know the nature of the sebum and the relative proportions of the lipids of which it is composed. The mixture to be analyzed can be removed from the substrate by means of a solvent. It can also be removed more conveniently by means of a plane support, such as a small glass square. However, in the latter case, it is nevertheless necessary to use a solvent in order to transfer the sample removed by the plane support into the said solvent, by dilution. The volume of solvent used must be large if it is desired to recover all the sample, which is essential if it is desired to determine the relative proportions of the constituents of the mixture. Thus, a solution is obtained which has an extremely low concentration of solute, and it is consequently necessary to concentrate it before depositing it on the chromatography plate. All these dilution and concentration steps complicate the chromatographic analysis and prove expensive in terms of time and skilled personnel.
Another disadvantage of analysis by thin layer chromatography is the small size of the samples subjected to analysis, which are generally between 1 and 5 microliters and may at the extreme limit be as much as 20 microliters. In fact, the chromatographic separation of the constituents of a mixture is all the more successful, the smaller the amount of mixture to be analyzed; however, this small size of the samples is a source of difficulty if it is desired to carry out a precise quantitative analysis of the constituents of the mixture.
Plates have recently been marketed which possess a layer of diatomaceous earth as the concentration zone, where the samples are deposited. These plates make it possible to deposit a larger amount of sample, which can be as much as about 500 microliters, without detracting from the quality of the separation. Nevertheless, chromatography plates of this type, like the conventional chromatography plates, requires a step for dilution, with a solvent, of the sample removed by a plane support, followed by a step for concentration of the solution obtained.